Its been a long while since I played 1st Ed but think they had differnt types of poison, with some having delayed effects etc. As DM you can decide to use the poison effect however you wish, I think your 25% idea is a good one, you could describe the players as feeling progressivley weaker as the poisn takes hold.
The poison's can have different effects. Introduce a Cleric/Druid scroll as a treasure item before they encounter the poison and make sure it has "Slow poison" on it. It should make them sweat some but not kill them outright. If someone does eventually die, make sure there is a Cleric back in town that can raise them. It's costs money but hey, you gotta suck up all the gold pieces they find some how.
I played 1st ed D&D in high school about 30 years ago. Now, my kids have taken an interest, so I've started DM-ing for them.
I first attended GenCon during their last year at UW Parkside, then in Milwaukee. I haven't gone in 20 years, but now my kids are talking about attending that, too.